There are three types of oxygenators in prevalent use today; the bubble oxygenator, the surface filming oxygenator and the membrane oxygenator. Artificial oxygenators have been highly successful in providing the human respiratory function in a cardiopulmonary bypass employed, for example, during open-heart surgery. The present invention relates primarily to oxygenators of the bubble type, though certain features thereof may be applicable to one of the other types of oxygenators.
There are a plurality of variables relevant to respiratory gas dispersion into venous blood which are considered in the design of the primary features of the present invention. It is believed helpful to identify these variables and evaluate their effect with respect to the blood oxygenating and carbon dioxide release process of the pulmonary function.
The first of these variables is the blood flow rate into the gas exchange system. The blood flow rate into an extracorporeal respiratory gas and blood exchange system is primarily at rates less than seven liters per minute, although the rate varies from one patient to another and, in fact, varies in the same patient depending upon other physiological conditions.
A second variable is the respiratory gas mixture, i.e. the percentage of oxygen and carbon dioxide flowing into the system. A mixture of 98% oxygen and 2% carbon dioxide is often used, however, other mixtures can be used depending upon other variables in the system. The appropriate ratio is determined by sampling the patient's blood and measuring the gas content of the blood after processing so that if excessive or insufficient amounts of carbon dioxide are removed from the blood, this can be adjusted.
The respiratory gas flow rate is another variable and is often in the range of 1 to 5 parts of respiratory gas to 1 part of blood for the process. If the blood is excessively or insufficiently saturated with oxygen, adjustments are made in the quantity of respiratory gas mixture introduced into the oxygenator.
Still another variable is the time period during which blood and oxygenating gas are exposed to one another. The longer the time of contact, the more absolute the maximum exchange of blood with the gas. Gas exchange may be defined as the release of excess of carbon dioxide from the blood and the addition of sufficient oxygen to the blood to change it from a venous to an arterial state. Moreover, carbon dioxide is twenty times more soluble in blood than oxygen so that the longer the time of blood-gas contact, the greater will be the saturation of the blood with carbon dioxide.
A further variable is the thickness of the film of blood around each aliquot of respiratory gas. Efficiency or rate of blood-gas equilibration in an extracorporeal blood respiratory gas exchange system is highly dependent upon the thickness of the blood film around each aliquot of respiratory gas. For example, a liter of venous blood with a few marble sized bubbles of gas will have a very thick blood cell between the gas bubbles. Therefore, a great amount of time would be necessary for the gas of these few bubbles to penetrate the diffusion gradients of the blood and equilibrate. The greater the number of bubbles for a given volume of blood, the less the distance that the gas must penetrate to equilibrate.
The relative diameter of each blood bubble is important to the exchange process in the following manner. By progressively making smaller bubbles with a given aliquot of gas, the surface of exposure of the blood to the gas phase is markedly increased. Moreover, as more smaller bubbles are introduced into the system for a given volume of blood, the thickness of the blood between the bubbles decreases and the rate of respiratory exchange increases as the diffusion distance for the gas through the blood has progressively diminished.
As the bubbles become extremely small, the ratio of the surface of that bubble to its volume becomes very large. As the surface of the tiny bubble represents a finite quantity of blood, containing respiratory gases, the gases will rapidly equilibrate between the blood and the gas within the bubble. As mentioned, carbon dioxide is greater than twenty times more soluble in the blood than oxygen. As the available volume of the bubble is small compared to the surface film, the partial pressures of gas stabilizes with the extraction of an insufficient amount of the carbon dioxide. As oxygen is absorbed from the bubble into the blood, the volume of the bubble decreases to the extent of the lost oxygen, and increases to the extent of the released carbon dioxide. But the percentage of carbon dioxide in the respiratory gas is about only 2%. Therefore, extremely small bubbles tend to retain carbon dioxide in excess of the demand and therefore there is an ultimate and optimum bubble size generally avoiding microbubbles.
One of the objects of the present invention is to provide an oxygenator that optimizes bubble size to effect the most efficient blood-gas exchange.
According to the LaPlace equation (T=k.sub.PR), the tension on the wall of a plastic (or non-rigid) sphere is in direct proportion to the pressure in the sphere and its diameter. Therefore, a bubble with a constant pressure develops an increasing tension on its walls as its diameter increases. If the diameter is made to increase excessively, the tension produced by the surface tension of the fluid content of the blood and its own elasticity increases to a point that it cannot maintain its integrity and the bubble ruptures. Therefore, larger bubbles are easier to remove from the system than smaller bubbles as the larger bubbles tend to destroy themselves.
Thus, it may be desirable to have smaller optimum bubbles at the point of oxygenation within the oxygenator and larger bubbles that may be easily ruptured at the point in the system where bubble collection and defoaming occur. The initial diameter of the bubble is influenced by the size of the aperture and rate of the introduction of respiratory gas into the blood. Also, a bubble rising in a column of blood (and bubbles) increases in size due to the decreasing hydrostatic pressure.
The temperature of the blood influences the reaction between the blood and the respiratory gases. As the temperature decreases from normal, the blood will be saturated at progressively lower partial pressures of the oxygen. The tissues require less oxygen for the metabolic function as temperature decreases. If saturated cold blood is supplied with a lower partial pressure pO.sub.2, the pressure differential between the blood and the tissues is minimized and the oxygen becomes less available to the tissues. Warm blood supplied in a saturated condition has a much higher partial pressure pO.sub.2, and, therefore, a greater partial pressure difference between the tissues and the blood, and a greater tendency for the flow of oxygen from the blood to the tissues. For these reasons, an efficient temperature control system is highly desirable in an oxygenator.
It is a primary object of the present invention, with these variables in consideration, to provide a bubble oxygenator having optimum bubble size, gas diffusion, debubbling and defoaming, and planar flow temperature control of the blood.